Aliphatic polyisocyanates which contain biuret groups have become of considerable commercial importance in the field of films and coatings. They are prepared from aliphatic diisocyanates and water, compounds from which water can be split off or compounds which are capable of liberating an amino group under certain conditions. Polyisocyanates which contain biuret groups can also be prepared by the direct reaction of amines and polyamines with diisocyanates.
It is also known that the biuret group is comparatively unstable under the conditions of the biuretization reaction so that the monoamine or polyamine used as starting material very readily changes into the corresponding isocyanate by "transbiuretization". Thus, for example, simply by reacting a high boiling diisocyanate with a monoamine it is possible to obtain the monoisocyanate corresponding to the monoamine by distillation. A polyisocyanate which contains biuret groups remains behind as distillation residue.
As is to be expected, this reaction also takes place when a diamine is reacted with excess diisocyanate to form urea and the reaction product is then biuretized. Here again, an equilibrium reaction takes place with the diisocyanate which is usually used in excess, to form the diisocyanate corresponding to the diamine put into the reaction. When the reaction mixture is subsequently purified by distillation to isolate the biuret polyisocyanate, the distillate contains not only the diisocyanate originally put into the process but also a second diisocyanate corresponding to the diamine put into the process. Alternatively, if a high boiling diamine is used, distillation of the diisocyanate originally put into the process results mainly in solutions of the biuret polyisocyanate in the diisocyanate which corresponds to the diamine. Thus, for example, when the method of preparation described in Example 13 of German Offenlegungsschrift 2,261,065 is carried out, the reaction of hexamethylene diisocyanate with 3,3'-dimethyl-4,4'-diamino dicyclohexyl methane results not only in the biuret polyisocyanate specified there but, in addition, more than 10% by weight of 3,3'-dimethyl-4,4'-diisocyanato-dicyclohexyl methane. This can easily be demonstrated, for example, by gas chromatographic analysis. Similar results are obtained when other examples described in German Offenlegungsschrift 2,261,065 are repeated, that is to say a monomeric diisocyanate corresponding to the diamine is formed in addition to the biuret polyisocyanate.
More recent tests have shown that when biuret polyisocyanate mixtures which can easily be produced with a maximum free diisocyanate content (for example, hexamethylene diisocyanate) of 0.7% are subjected to prolonged storage, in particular under uncontrolled conditions, for example, when transported by sea in hot climates, the proportion of monomeric diisocyanate is liable to increase again over a period of a few months and may rise to over 1%. This is due to catalytic reactions with the walls of glass or metal containers and possibly also due to catalytic effects of impurities which cannot be identified analytically, and occurs particularly at elevated temperatures.
When products prepared from a diamine and a diisocyanate (e.g. according to German Offenlegungsschrift 2,261,065, Example 13) have been stored for a long time or at elevated temperatures, e.g. at 50.degree. C., they are found to contain two monomer diisocyanates:
(1) More than 10% by weight of the diisocyanate already mentioned above which corresponds to the diamine used as starting material; and PA0 (2) About 1 to 2% by weight of the diisocyanate used as starting material. PA0 o represents 0 or 1, PA0 m is 1 or 2 when o = 0 and 1 when o = 1, PA0 n represents 1 or 2 when o = 0 and 1 when o = 1 such that the sum of n + m is always 2 or 3, PA0 R represents PA0 X represents a group of the formula --NH--CO--O-- or --NH--CO--, the amino group --NH.sub.2 being in all cases linked to the nitrogen atom of the said group to form the structure of a hydrazine derivative. PA0 N-(2-aminopropyl)-ethanolamine; PA0 N-(6-aminohexyl)-aminoethanol; PA0 N-(6-aminohexyl)-1(or -2)-methyl-ethanolamine; PA0 N-(3-amino-1,5,5-trimethyl-cyclohexylmethyl)-aminoethanol; PA0 N-(3-amino-1,5,5-trimethyl-cyclohexylmethyl)-1(or -2)-methyl-ethanolamine; PA0 N-(3-amino-1,5,5-trimethyl-cyclohexylmethyl)-1,1 (or -2,2)-dimethyl-ethanolamine and PA0 N-(3-amino-1,5,5-trimethyl-cyclohexylmethyl)-1,2-dimethylethanolamine. PA0 N-aminoethyl-3-hydroxypropane carboxylic acid-1-amide; PA0 N-(4-aminobutyl)-5-hydroxypentane carboxylic acid-1-amide; PA0 N-(6-aminohexyl)-5-hydroxypentane carboxylic acid-1-amide and PA0 N-(3-amino-1,5,5-trimethyl-cyclohexylmethyl)-5-hydroxypentane carboxylic acid-1-amide. PA0 p preferably represents 2 or 3. PA0 q represents an integer of from 4 to 10, preferably 5.
Toxicological investigations and many years experience in the processing of biuret polyisocyanates and lacquer binders, have shown that there is no danger in using the lacquers mentioned above under the protective conditions normally employed for lacquer processing if the biuret polyisocyanates used in the process do not contain more than 0.7% of free diisocyanate (e.g. hexamethylene diisocyanate). The limit of 0.7% has been accepted in the memorandum "PUR-Anstrichstoffe" published by the Association of German Industrial and Trade Unions and in the "Polyurethane-Report" of the Paintmakers Association.
Apart from these toxicological considerations, the presence of a low boiling monomeric isocyanate in a polyisocyanate which is intended to be used for lacquers and coatings is a major disadvantage. Monomeric isocyanates are liable to evaporate with the solvent before the lacquer has completely hardened. This is liable to produce defects in the surface of the lacquers and occurs most commonly when rapid drying is carried out at elevated temperatures. On the other hand, it is often necessary to dry a lacquer under such stringent conditions because the reactivity of lacquer polyisocyanates which contain too high a proportion of isocyanates which are not more than difunctional is very low. High quality polyurethane lacquers are generally obtained from polyfunctional polyisocyanates.
A biuret polyisocyanate based on hexamethylene diisocyanate and stabilized against decomposition has been disclosed in German Offenlegungsschrift 2,437,130.
The stability of this polyisocyanate is obtained by the presence of N-formyl groups so that the polyisocyanate mixture contains a substantial proportion of the following compound: ##STR1##
The advantage of the high stability of this biuret polyisocyanate mixture is offset by various disadvantages in its practical application. These are due to the low isocyanate functionality. Since the product contains the difunctional compound shown above, the average functionality is less than 3. Lacquers produced from this product therefore harden only slowly and often have too little initial hardness.
The present invention makes it possible to obtain new biuret polyisocyanates which combine all the advantages of the known polyisocyanates which have have a biuret structure and, in addition, no longer have the disadvantage of decomposing into the original monomeric diisocyanate during prolonged storage. The new polyisocyanates with biuret groups contain urethane groups in addition to these biuret groups and are prepared by the reaction of certain compounds with amino and hydroxyl groups which will be defined in more detail below with excess quantities of simple diisocyanates.
The reaction of amino alcohols with aromatic and aliphatic diisocyanates and derivatives thereof is not new and belongs to the state of the art. Thus, for example, amino alcohols are commonly used chain lengthening agents for the production of polyurethane elastomers. In German Offenlegungsschriften 1,720,747; 2,031,408 and 2,242,520 there also mentioned, among others, addition products of up to 3 mols of aliphatic diisocyanate with amino alcohols, without these products being described in any detail. There is no evidence from these citations that the products mentioned there contain biuret groups or that they constitute polyisocyanates which are resistant to decomposition into the original monomers.
Moreover, it is clear from the comparison experiments described hereinafter that the amino alcohols mentioned in the aforesaid prior publications are not ideally suitable as starting materials for the preparation of biuret polyisocyanates.